Happy birthday today to Wes Montgomery. Any guitarist worth his or her salt has at least ONE album from Wes Montgomery, the master non-plectrist. No, I didn't invent that word, plectrist. It's actually derived from plectrum, which is what guitar nerds and speakers of dead languages call a pick. [Incidentally, another great guitarist, Billy Bauer, made a record in the 1950s called Plectrist. But I'm getting even more off topic here.]
Wes Montgomery, however, was a non-plectrist. He didn't use an external tool to pick the strings. He used his thumb. That's what makes Wes Montgomery's sound so identifiable - warm, casual, and about as 'natural' as an amplified electric instrument can sound without using algorithms or superhero powers.
Check out this video of Wes playing Thelonious Monk's "Round Midnight." About two minutes in, you'll see the closeup on his hand. Not plectacular, but spectacular. - Josh